1. Field of the Invention
The present invention relates to an apparatus for detecting an amount of strain, which is to be applied to measurement of pressure of fluid such as gas or liquid, and to a method for manufacturing such an apparatus.
2. Description of the Related Art
With respect to fabrication of sensing elements in the conventional strain detecting apparatus, there exists a technique to form poly-crystalline silicon films (strain gages) on a substrate sheet utilizing a plasma CVD method, as described in Japanese Patent Publication No. H6-70969.
In the conventional strain detecting apparatus fabricated utilizing the above-mentioned method, an electrical insulating layer 31 (i.e., a silicon oxide film) is formed on a diaphragm-main body 30, which has a cavity 30a into which fluid to be subjected to a pressure measurement is to be introduced, and then, the poly-crystalline silicon films are formed on the electrical insulating layer 31 so as to serve as the strain gages 32, as shown in FIG. 7.
More specifically, in such a strain detecting apparatus, the poly-crystalline silicon film is formed at a temperature of up to 590° C. utilizing the plasma CVD method. Such a poly-crystalline silicon film is subjected to a process such as photolithography to provide the strain gages 32 having an appropriate shape.
Such strain gages 32 are advantageous in manufacturing the poly-crystalline silicon films in large quantities and at low cost. The strain gage 32 has appropriate temperature characteristics for the strain gage, i.e., TCR (temperature coefficient of resistance) of from −700 ppm/° C. to −200 ppm/° C. and TCS (temperature coefficient of strain) of ±300 ppm/° C., thus providing good characteristic properties for the silicon film.
In the alternative method for forming poly-crystalline silicon films, an amorphous silicon film is formed at a temperature of up to 590° C. utilizing a plasma CVD method or a sputtering method, the amorphous silicon film is subjected to a crystallization process utilizing a laser annealing method, and then the resultant silicon film is subjected to a process such as photolithography to provide strain gages having an appropriate shape.
It is specifically noted that the thus provided strain gage is formed of substantially poly-crystalline silicon film having a relatively large grain size. It is also possible to reduce an amount of amorphous silicon left in the film to an excessively small amount.
However, the above-mentioned substantially poly-crystalline silicon film, which serves as the strain gage 32 in the strain detecting apparatus, has a grain size of about 0.1 μm, and is provided on its lower side with an amorphous interface-layer 33 coming into contact with the electrical insulating layer 31, as shown in FIG. 8. When a continuous measurement of strain was made in a relatively high temperature atmosphere (at least 100° C.) with the use of the strain detecting apparatus in which the above-mentioned substantially poly-crystalline silicon film is used as the strain gage, there observed a phenomenon (load characteristic at high temperature) in which the zero point for an output creeps as shown in FIG. 9. This occurs due to the fact that retarded elasticity of the amorphous interface-layer 33 causes the strain gage itself to creep in a stress applying direction (compression or tensile direction).
In addition, the laser annealing process has to be applied to each of sensing elements (i.e., the strain gages) to subject the above-mentioned amorphous interface-layer 33 to the crystallization process so as to provide the appropriate strain gages, thus being inconsistent with mass production. Further, the grain size of the silicon film exerts a significant influence on temperature characteristic of the strain gage. The silicon film, which has been subjected to the laser annealing process, has TCR (temperature coefficient of resistance) of about 2,000 ppm/° C. and TCS (temperature coefficient of strain) of −1,500 ppm/° C., thus being unsuitable for the strain detecting apparatus.